Rationalise WebMock test setup
Rationalise database_cleaner setup.
Return 400 when registering an artefact fails

Previously, the api would return a 200/201 even if the save failed due to validation errors.  This fixes that for API (i.e. JSON) requests.
Support registering travel-advice artefacts

These have slugs of the form travel-advice/<country_slug>, which required govuk_content_models to be updated before they were considered valid.
Use 422 not 400 when validation fails
Merge pull request #51 from alphagov/fix_artefact_registration

Fix artefact registration